Elemental carbon aerosols: Sources, long-term trends and implications to radiative forcing.

摘要

Elemental carbon aerosols (EC) play a key role in the earth's temperature regulation because of their abilities to absorb solar radiation. Unfortunately, the EC data needed to estimate radiative forcing is currently unavailable. Another complicating factor is the tropospheric residence time of ∼1 week for EC aerosols. Due to the short residence time, the EC concentrations show large variations, with distance transport from the emission sources. Hence, EC data that represent various regions are needed for determination of regional or global radiative forcing. In the United States there exists a large industrial base in the Midwest that also has a large population and transport sector. The industrial, vehicular and other sources result in a very large emission of EC and other pollutants. The prevailing west to east airflow results in transport of particulate matter and other pollutants from the Midwest to the Northeast. The EC emitted in the Midwest that is not deposited near the sources will be transported.; In this dissertation, the EC aerosols at Mayville and Whiteface Mountain, New York are investigated. A new technique has been developed to transfer EC from cellulose filter to quartz filter without loss in order to measure EC by the thermal-optical method. Daily samples for ten approximately two-week episodes over the years 1983 to 2002 were analyzed for EC. Based on the episodic results with sulfate concentrations it is shown that most of the EC aerosols in the rural Northeast can be attributed to transport from outside sources. Directional analysis suggests that the Midwest is the principal source region. Composite monthly averages were also determined for 1984, 1990, 1996 and 2002 at Mayville. A decreasing trend in EC concentrations is found over the past two decades. The EC aerosol direct radiative forcing in regional climate is evaluated. The trend in EC and SO42- concentrations cause a slight change in net aerosol direct forcing. The yearly net aerosol direct forcing by transport was 1∼4-fold that of the background. Thus the transported EC aerosols from outside sources especially the Midwest play an important role in the regional climate of the Northeast.